Electronic paper is one of novel and reusable electronic display devices, and has a shape of thin and flexible paper. Electrophoretic electronic paper is one kind of electronic papers, and displays images with the movement of charged particles comprising black particles and white particles upwards and downwards in applied electric fields. An active matrix driving technique is needed in order to display text information in an electronic paper. For example, thin film transistor (TFT) technique is one kind of techniques for active matrix driving.
Electrophoretic electronic paper is generally made by assembling an active substrate with an electrophoretic substrate to form a cell. The electrophoretic substrate is provided with a common electrode thereon, and is also coated with an electrophoretic particle layer; and the active substrate functions to input data signals to control images to be displayed in real time, and is formed of a plurality of pixel units arranged in a matrix.
The electrophoretic electronic paper has several advantages in that, for example, it has better visual properties such as contrast and brightness, low power consumption, and light weight easily for a thin profile, and can be formed into varieties of shapes. The electrophoretic electronic paper also has some disadvantages in that, for example, an electrophoretic electronic paper display panel needs a relatively high voltage, and a high voltage may result in a large leakage current. Therefore, in general, a large storage capacitor should be designed in order to keep charges stable.
In order to realize large storage capacitors, commercially available active electronic paper display panels need the storage capacitor to occupy nearly the whole of a pixel unit. This is because the electronic paper film is relatively thick, which makes the capacitance formed between a pixel electrode layer and a common electrode layer of the electrophoretic substrate small, so that a capacitor has to be formed between source-drain electrodes and the common electrode layer on the active substrate. That is, the capacitor formed between the source-drain electrodes and the common electrode layer on the active substrate becomes a main capacitor of each pixel, but the capacitor formed between the pixel electrode layer and the common electrode layer of the electrophoretic substrate may be neglected.
A current electronic paper has generally a resin passivation layer besides a passivation layer to reduce parasitic capacitance, so that the pixel electrode layer may be spread in a larger area. FIG. 1 shows a cross-section structure of the active substrate in a pixel (sub-pixel) structure of the current electronic paper. It can be seen from FIG. 1 that, in order to achieve a relatively large storage capacitor, a source-drain electrode 5 is spread in a large area in a pixel region. It can be known from the capacitance equation, C=∈S/d (wherein “∈” is a dielectric constant of the dielectric between plates of a capacitor, “S” is an area in which the parallel plates face each other, and “d” is a distance between the parallel plates), that: the larger the ∈, the larger the capacitance; the larger the area, the larger the capacitance; and the smaller the distance, the larger the capacitance. In the active substrate, the source-drain electrode 5 and the common electrode 3 correspond to the two plates of the capacitor, and a gate insulating layer 4 functions as an insulating layer. In the capacitance equation, since ∈ is in connection with the insulating material filled between the two plates of the capacitor, ∈ is substantially constant when the insulating material is selected. Therefore, in order to achieve large capacitance in each pixel of the active substrate, one measure is to increase the area in which the source-drain electrode faces the common electrode. However, this measure has less effect since the pixel area itself is limited. Another measure is to reduce the thickness of the gate insulating layer. However, if the gate insulating layer is too thin, short or breakdown between edges of the gate electrode and the source-drain electrodes might happen, and the properties of TFT (thin film transistor) might be influenced negatively.
Therefore, a problem to be solved at present is to increase storage capacitance effectively without influencing the properties of TFT devices.